The progressivity of the U.S. tax system has dramatically declined over the past seven decades. The upshot is that for most income levels, the U.S. tax system now resembles a flat tax that becomes regressive at the very top end, meaning the super-rich pay proportionately less. Today, virtually all income groups pay roughly 28 percent of their income in taxes--– except for the 400 richest Americans.

As President Obama and his campaign team know, the Buffett Rule -- officially the Paying a Fair Share Act -- is a powerful symbol of many Americans' desire for economic justice and reprisal against the richest 1 percent that has arguably cached the country's wealth for itself.  So it's only rational that Obama would bring it into focus as the primaries shift to the two-man  race for the presidency.